[racket] a little macro exercise

I've been feeling pretty good about my macro-fu recently so I gave it a shot
(I havent looked at anyone's solutions yet). This seems to pass all the test
cases but I dont think it passes the "linear" requirement yet.

(define-syntax (cas-cad-e stx)
 (syntax-case stx ()
   [(_ e ((v) exp ...))
    (with-syntax ([break (datum->syntax stx 'break)])
      #'(call/cc
         (λ (break)
           (if (equal? e v)
               (begin exp ...)
               (void)))))]
   [(_ e ((v1) exp1 ...) ((v2) exp2 ...) ...)
    (with-syntax ([break (datum->syntax stx 'break)])
      #'(call/cc
         (λ (break)
           (if (equal? e v1)
               (begin exp1 ... exp2 ... ...)
               (cas-cad-e e ((v2) exp2 ...) ...)))))]))





On Fri, Oct 8, 2010 at 9:04 PM, Shriram Krishnamurthi <sk at cs.brown.edu>
wrote:
> One of my students recently sent me this needless email message:
>
>> Well, how would you do switch fall-through in Scheme? Could you
>> write a version of the case statement that does that?
>
> Since the honor of Racket was at stake (yes, we can have all the same
> stupid features the scripting languages have!), I wrote down the code
> for this, and realized it would make a cute little macro exercise.
>
> Spec: define a case construct syntactically just like that of Racket.
> In terms of semantics:
>
> - each branch automatically falls through to the next,
>
> - the last one returns its answer since it has no next clause, and
>
> - any branch can contain (break <expr>), which evaluates <expr> and
> returns its value as that of the entire case.
>
> In honor of its behavor, we'll call this cas-cad-e.  Thus,
>
> (define (cas1 v)
>   (cas-cad-e v
>            ((1) (display "1"))
>            ((2) (display "2") (break 2)
>            ((3) 3))))
>
> (cas1 1) ==> 2       (and prints "12")
> (cas1 2) ==> 2       (and prints "2")
> (cas1 3) ==> 3       (and prints nothing)
> (cas1 4) ==> <void>  (and prints nothing)
>
> If anyone wants to look at my solution, here it is:
>
>  http://github.com/shriram/cas-cad-e
>
> Of course, those who know how to do this don't need to peek, and those
> who don't shouldn't.
>
> Shriram
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